1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer, and more particularly to an image forming apparatus that adds gloss to an image formed on recording paper.
2. Description of the Related Art
As a conventional electrophotographic image forming apparatus, for example, there has been proposed a method for adding gloss to recording paper by disposing a second fixing device on a downstream side of a first fixing device and heating the recording paper passing through the first fixing device again at the second fixing device (as discussed in Japanese Patent Application Laid-Open No. 63-192068).
However, this method can only add gloss to a whole surface of the recording paper and not in just a limited area. In addition, the method adds heat even to a portion that requires no gloss addition, and hence a problem of increased power consumption is created.
Thus, there has bee proposed a method for adding gloss to the recording paper only in a limited area by disposing a thermal head as the second fixing device on the downstream side of the first fixing device and heating an arbitrary area of the recording paper again by the thermal head (as discussed in Japanese Patent Application Laid-Open No. 2007-52175).
However, when gloss is added to the recording paper by using the thermal head, a sudden change in a number of heating units of the thermal head to be powered-on causes fluctuation in voltage applied to the thermal head. As a result, a heat generation amount of the thermal head fluctuates. In such a case, uneven brightness occurs on the recording paper, causing deterioration of image quality.